The present invention relates to a driving method of an active matrix type liquid crystal display which uses, as pixel drive elements, active elements made of a ferroelectric material.
At present, matrix type liquid crystal displays are mainly used in which pixels are arranged in a matrix form. And the matrix type liquid crystal displays are classified into the simple matrix type and the active matrix type in terms of the driving method.
The active matrix type liquid crystal display has a configuration in which memory elements each consisting of a capacitor and a nonlinear resistor element such as a diode or a transistor are connected to respective pixels. The capacitors are stored with charge while the nonlinear resistor elements are caused to operate in accordance an input signal. The display continues to operate by virtue of the charge stored in the capacitors even after the input signal disappears, thus maintaining contrast in approximately the same level as that obtained by static driving. For this reason, the active matrix type liquid crystal display is now widely used with its increased display capacity.
The thin-film transistor (TFT) is most commonly used as the active element, although the diode and the MIM (metal-insulator-metal) element are also used.
FIG. 9 is a sectional view conceptually showing a structure of an active matrix type liquid crystal display using thin-film transistors. A thin-film transistor (TFT) portion consists of a gate electrode 12 formed on a glass substrate 11, a gate insulating film 13 formed so as to cover the gate electrode 12, a channel 14 made of amorphous silicon (a-Si) and formed over the gate electrode 12, and a source region 15 and a drain region 16 formed on the channel 14 on its both sides. An pixel electrode 19 is connected to the TFT portion, to constitute a bottom substrate A. On the other hand, a top substrate B is constituted of a glass substrate 20 and a scanning electrode 21 made of a transparent metal and formed on the glass substrate 20. A liquid crystal C is interposed between the bottom substrate A and the top substrate B, to constitute a liquid crystal element. A plurality of liquid crystal elements each having the above structure are arranged in a matrix form, to constitute an active matrix type liquid crystal display.
In this active matrix type liquid crystal display using thin-film transistors, image information (an input signal) applied to the source electrode 17 is transmitted to the liquid crystal C (interposed between the pixel electrode 19 and the scanning electrode 21) via the channel 14 that is on/off-controlled by a voltage applied to the gate electrode 12, and stored as charge by a capacitance of the liquid crystal C. However, the charge held by the liquid crystal C decreases with time because of leakage in each liquid crystal C itself, a leak current in the thin-film transistor, and other factors. Therefore; the contrast of a displayed image likely lowers with time.
The above type of liquid crystal display also has a problem that due to a complex process of forming the thin-film transistors the yield tends to be low in producing a large-size liquid crystal display.
To solve the above problems, it has been proposed that a ferroelectric material be used as the active elements instead of thin-transistors, to realize liquid crystal displays capable of producing high-quality images with a simple structure and a reduced number of production steps (for instance, Japanese Unexamined Patent Publication No. Sho. 64-4721).
FIG. 1 shows a sectional view of a one-pixel portion of an active matrix type liquid crystal display using, as drive elements, active elements made of a ferroelectric material. FIG. 2 is a top view of a bottom substrate A.
The bottom substrate A is constituted as follows. A image electrode 2, which receives image information, is formed on a glass substrate 1. A ferroelectric material layer 3 is formed over the entire pixels. Further, a pixel electrode 4 is formed on the image electrode 2 and the ferroelectric material layer 3. The ferroelectric material layer 3 may be made of a ferroelectric material selected from perovskite materials such as TiBaO.sub.3, PhTi and WO.sub.2, Rochelle salt, tartrates, phosphates, arsenates, alkali metal dihydrogen phosphates such as KDP, guanidine type materials such as GASH and TGS, amorphous materials of LiNbO.sub.3, LiTaO.sub.3, PbTiO.sub.3, etc., polymers such as PVF.sub.2, TrFE and a copolymer thereof, and single crystals and polycrystals of B.sub.14 Ti.sub.3 O.sub.12. A top substrate B is constituted of a glass substrate 5 and a scanning electrode 6 made of a transparent metal and formed on the glass substrate 5. A liquid crystal C is interposed between the bottom substrate A and the top substrate B, to constitute a single pixel portion of the liquid crystal display.
The above active element, which serves as a drive element of the active matrix type liquid crystal display, utilizes the residual polarization phenomenon in which even after application of an electric field to a ferroelectric material is finished, an electric field caused by residual polarization remains therein and the residual polarization is erased by applying a counter electric field of opposite polarity.
Referring to FIG. 6, the electric field vs. charge density characteristic of a ferroelectric material will be described. In FIG. 6, the horizontal axis and the vertical axis represent an electric field strength E applied to a ferroelectric material and a charge density P stored in the ferroelectric material, respectively.
The charge density P increases as the electric field E is Increased. Even after application of an electric field (Eo) to the ferroelectrlc material is finished, charge called residual polarization Pr remains therein, to cause an internal electric field Ec in accordance with the density and polarization of the residual charge. If a counter electric field -Ec opposite in polarity to the residual polarization and having a magnitude for neutralizing it is applied externally, the residual polarization disappears. If an electric field opposite in polarity to and larger than the residual polarization is applied externally, charge that is opposite in polarity to the previously created charge is generated and residual polarization -Pr remains after application of the electric field -Eo is finished. As a result, an internal electric field opposite in polarity to the previous one occurs in accordance with the density of the residual charge thus generated.
The electric field that develops in accordance with the residual polarization Pr or -Pr can be applied to the liquid crystal that is connected in series to the ferroelectric material.
FIG. 3 shows an equivalent circuit of the above liquid Crystal display. In FIG. 3, symbol P.sub.mn represents an element (pixel) that in a series connection of a capacitance component C.sub.LC Of a liquid crystal portion 30 adjacent to both of the pixel electrode 4 and the scanning electrode 6 (portion of j .times. k in FIG. 2) and a capacitance component C.sub.FE of a ferroelectric material portion 40 adjacent to both of the image electrode 2 and the pixel electrode 4. Scanning electrodes of the respective sets of pixels P.sub.11 -P.sub.in, P.sub.21 -P.sub.zn and P.sub.ml -P.sub.mn are indicated as scanning lines a.sub.1 -a.sub.m, and image electrodes of respective sets of pixels P.sub.11 -P.sub.ml, P.sub.12 -P.sub.m2 and P.sub.in -P.sub.mn are indicated as image signal lines b.sub.1 -b.sub.n. The scanning lines a.sub.1 l-a.sub.m and the image signal lines b.sub.1 -b.sub.n constitute a matrix.
FIG. 4 shows a driving method proposed for the active matrix type liquid crystal display using ferroelectric material active elements. In FIG. 4, symbols a.sub.1 -a.sub.m represent scanning signals to be applied to the respective scanning lines given the same symbols in FIG. 3, and symbols b.sub.1 -b.sub.n represent image signals to be applied to the respective image signal lines given the same symbols in FIG. 3.
Pixel rows to which image information is to be written are selected by sequentially applying scanning signals having a scanning voltage +Vs or -Vs to the scanning lines a.sub.1 -a.sub.m. Image signal data are supplied to respective pixels connected to the scanning line to which a scanning signal is being applied by applying image signals having an image voltage +Vd or -Vd to the image signal lines b.sub.1 -b.sub.n.
When the scanning line a.sub.1 is selected in a period T.sub.1 of the first field by application of a voltage +Vs, a voltage -Vd is applied to the image signal line b.sub.2 for the pixel P.sub.12 which should be ON among the pixels connected to the scanning line a.sub.1 and a voltage +vd is applied to the image signal lines b.sub.1 and b.sub.n for the pixels P.sub.11 and P.sub.in which should be OFF. Signal processing of the first field continues while the other scanning lines a.sub.2 -a.sub.m are sequentially selected in the similar manner. Subsequently, the second field scanning is performed.
In the second field, voltages -Vs are sequentially supplied to the scanning lines a.sub.1 -a.sub.m to be selected, and a voltage +Vd is applied to image signal lines for pixels which should be ON and a voltage -Vd is applied to those for pixels which should be OFF.
In the period T.sub.1 shown in FIG. 4, among the pixels P.sub.11 -P.sub.in that are connected to the scanning line a.sub.1 to which a scanning voltage +Vs is applied, the display pixel P.sub.12 (associated liquid crystal element should be made ON) that is connected to the image signal line b.sub.2 to which an image voltage -Vd is applied receives, in effect, a selection voltage V(selection) that is a sum of the scanning voltage Vs and the image voltage Vd. On the other hand, the non-display pixels P.sub.11 and P.sub.in (associated liquid crystal elements should be made OFF) that are respectively connected to the image signal lines b.sub.1 and b.sub.n to which an image voltage +Vd is applied receive, in effect, a non-selection voltage V(non-selection) that is a difference between the scanning voltage Vs and the image voltage Yd.
The pixels P.sub.21, P.sub.22, . . . , P.sub.mn connected to the scanning lines a.sub.2 -a.sub.n which is not supplied with a scanning voltage Vs and is therefore at the 0 level receive, in effect, a scanning line non-selection signal V(non-selected line) that is equal to the image voltage +Vd or -Vd.
The ferroelectric material portion 40 of each pixel is supplied with a voltage that is proportional to a ratio of the capacitance C.sub.LC of the liquid crystal portion 30 to the capacitance C.sub.FE of the ferroelectric material portion 40.
Therefore, the voltage V.sub.FE across the ferroelectric material portion 40 of the display pixel P.sub.12 which receives the selection voltage V(selection) is given by ##EQU1##
The voltage V.sub.FE across the ferroelectric material portion 40 of the non-display pixel P.sub.12 or P.sub.in which receives the non-selection voltage V(non-selection) is given by ##EQU2##
Further, the voltage V.sub.FE across the ferroelectric material portion 40 of each of the pixels P.sub.21, P.sub.22, . . . , P.sub.mn which receives the scanning line non-selection voltage V(non-selected line) is given by ##EQU3##
As already described above, FIG. 6 shows the electric field vs. charge density characteristic of a ferroelectrlc material used in the above active matrix type liquid crystal display.
With the progress of the sequential scanning operation, when the application of the voltage V.sub.FE to the ferroelectric material portion 40 of each respective pixel is finished, an internal electric field remains in the ferroelectric material portion 40 due to residual polarization Pr that is proportional to the applied voltage V.sub.FE. The internal electric field causes a voltage V.sub.REM, which is proportional to the voltage V.sub.FE, to be applied to the liquid crystal portion 30.
Referring to FIG. 5, a description will now be made of an electro-optical characteristic, i.e., a relationship between a voltage applied to a liquid crystal element and a light transmittance thereof. More specifically, a characteristic curve of FIG. 5 shows a relationship between an effective voltage V applied between the pixel electrode and the scanning electrode of a liquid crystal element and a light transmittance of the liquid crystal element. An effective voltage at which the liquid crystal element exhibits a transmittance of 50% is defined as an operation threshold voltage Vth. Around the threshold voltage Vth, the transmittance varies relatively steeply with the voltage applied to the liquid crystal element. That is, the transmittance varies with the voltage applied to the liquid crystal element. Therefore, in driving pixels that are arranged in a matrix form, in which case divided voltages are applied to pixels other than display pixels, crosstalks may occur to lower the contrast.
Referring to FIG. 4, when the voltage V.sub.s applied to the scanning line a.sub.1 disappears after the period T.sub.1, the residual polarization Pr generated by the divided voltage V.sub.FE remains in the ferroelectric material portion 40, and the residual polarization Pr causes the residual voltage V.sub.REM to develop across the ferroelectric material portion 40.
The voltage V.sub.REM is applied to the liquid crystal portion 30. A pixel that was given the selection voltage V(selection) which makes the voltage V.sub.REM exceed the operation threshold voltage Vth is made ON. On the other hand, in a pixel that was given the non-selection voltage V(non-selection) which makes the voltage V.sub.REM smaller than the threshold voltage Vth, the liquid crystal element is rendered, in effect, in a non-operating state (OFF), because its transmittance is smaller than 50%. Similarly, in a pixel that was given the scanning line non-selection voltage v(non-selected line) which makes the voltage V.sub.REM much smaller than the threshold voltage Vth, the liquid crystal element is also made OFF.
Then, in the period T.sub.2, a scanning voltage Vs is applied to the scanning line a.sub.2, and an image voltage +Vd or -Vd is applied to the image signal lines b.sub.1 -b.sub.n (see FIG. 4). As a result, the pixel P.sub.2n receives the selection voltage V(selection) and the pixels P.sub.21 and P.sub.22 receive the non-selection voltage V(non-selection). The selected pixel P.sub.2n is rendered in an operating state by the residual voltage V.sub.REM developing in the ferroelectric material portion 40.
Subsequently, the remaining scanning lines are sequentially scanned to form a one-field image.
In the next one field, the scanning signal and the image signal have voltages that are opposite in polarity to those in the first field, and the respective pixels operate in the same manner as in the first field while receiving voltages of opposite polarity.
Subsequently, the polarities of the respective voltages are reversed every field.
However, even with the above setting of the voltages, since the transmittance of the liquid crystal element varies relatively steeply with the voltage applied thereto around its electro-optical threshold voltage, crosstalks may occur. Further, if voltages that are applied to liquid crystal portions when the selection voltage V(selection) is applied to pixels are much higher than the threshold voltage Vth, a flicker likely occurs.